Geology of the Hauraki Plains
The Hauraki Plains have evolved to their present formation over millions of years. In the late Miocene, a rift valley formed 25 kilometres wide and over 200 kilometres long - the crust slumped 500 to 2000 metres.
The resulting valley filled, over time, with pumice, mud and gravel. For a considerable period of time (~1 million years), the Waikato River, which has changed its course a number of times throughout its history, flowed into the Hauraki Gulf, and it brought with it great volumes of pumice, gravel and ash from the central volcanic plateau.
The Miocene was a violent period of time for the central volcanic plateau, which produced a great deal of material and the pumice, mud and gravel deposits from the Waikato River filled the Hauraki rift valley to form a shallow basin, until the Oruanui eruption at Taupo 27000 years ago caused the outlet from the lake to be blocked and the water level rise 120 metres to find a new outlet.
Around 22000 years ago, the blockage at the original outlet of Lake Taupo gave way and ~80 cubic kilometres of water and debris flowed down the river valley ultimately causing the Waikato River to change course towards Cambridge and no longer emptying into the Hauraki basin via the Hinuera Valley.
Taupo eruptions also contributed a considerable quantity of airborne debris in the form of pumice and ash over 200 metres deep over the whole of the North Island.
For the past 10000 years, during the Holocene, the basin continued to accumulate estuarine and river deposited sediment and began the process of expansion into the Firth of Thames. This expansion continues today with silt and sediments carried into the Firth of Thames principally via the Waihou and Piako Rivers. Ancient coastlines have been identified as 1250 years ago through Orongo (~4km from the present coastline) and 6500 years ago at Kerepehi (~12km from the present coastline).
Waihou Journeys: The Archaeology of 400 Years of Maori Settlement by Caroline Phillips
Transactions and Proceedings of the Royal Society of New Zealand 1868-1961 - Notes on the Waikato River Basins by L. Cussen
Wikipedia - Hauraki Plains, Waikato River
Siltation & Sedimentation
Siltation is the pollution of water with fine particulates, usually erosional, and mostly made of fine silt or clay particles. It refers both to the increased concentration of suspended sediments, and to the increased accumulation (temporary or permanent) of fine sediments on bottoms where they are undesirable. Siltation is most often caused by soil erosion or sediment spill.
The process of suspended material settling out of fluid is termed sedimentation - at least sedimentation describes the final result of the suspended material coming to rest and building up on the bottom or against an object. While there are quite precise equations and calculations that can be performed, in simple terms, particles will remain suspended in water while there is a minimum amount of flow and turbulence in the fluid, and sedimentation will only occur when the flow and turbulence reduce enough that the particles are able to settle.
Siltation is often caused by human activity, where modification and development of land surrounding a waterway takes place, including the drainage of once wet or swampy land. While this has taken place within the last 150 years, the Hauraki Plains have been acumulating silt and mud deposits for at least 10000 years, so it can be assumed that whether or not human activity has contributed to the present siltation and sedimentation of the Waihou River and the Firth of Thames, there is considerable debris that makes its way into the river from natural erosion.
This Earthcache
This is an earthcache and like any earthcache, the purpose of it is to provide a geological lesson at the site you are visiting. This is not a virtual cache and you must visit the site, perform an experiment, and answer the questions below in order to claim you have 'found it'. Please note that you will not be able to complete this earthcache in one day - it will take at least two weeks to complete it (hence the high difficulty rating).
Before claiming your find, you must submit your answers for the questions below to the owner of this earthcache, by clicking on the link to the owner's profile at the top of the page, and sending the owner an email.
When emailing your answers, don't forget to include the name of this earthcache, and choose the option to include your email address so that we can reply to you. Do not include answers in your log on the cache page.
After you have emailed your answers, you can go ahead and log your find. Any problems with your answers, and we'll get back to you.
1. At the published coordinates, you will be standing on a small jetty on the edge of the Waihou River. Since this earthcache is all about silt, you will need to quantify the silt in the water. Normally this would be done by weight using very fine filters to collect the silt. We will be looking for a volumetric measure by following this procedure:
a. At the published coordinates, take a sample of the water by immersing a bottle into the water. You should be taking a sample of at least 1 litre of water.
b. At home, vigorously 
shake the water to make sure all the silt is suspended and then transfer the water into flat bottomed glass or clear plastic jar/bottle with straight sides.
c. Place the bottle in a stable location where you can access it to make measurements but it will otherwise be undisturbed. Ensure that your jar/bottle is tightly sealed so that no water will evaporate off.
d. Measure the height of the water level, and subtract the thickness of the bottom of the jar. This measurement, in millimetres, is your reference level - provide this measurement to the cache owner. This measurement should be at least 200 millimetres - if it's less than this then you should choose a skinnier and taller vessel for the experiment.
e. Each day, at the same time of day, measure the thickness of the silt sediment that has formed on the bottom of the jar, once again making sure to subtract the thickness of the bottom of the jar. Calculate the % by volume of the silt that has precipitated out of the water by dividing this measurement by the reference measurement (from step d) and multiplying by 100. At the same time as each measurement is taken, observe and write down the clarity of the water. Provide to the cache owner the 14 measurements and calculated volume percentages along with your descriptions of the clarity of the water each day.
2. Record the time of day and date of when your water sample was taken and provide the cache owner with that information along with a description of the water colour and the direction and strength of the tidal current at this point. Hint: North is to the left when facing the river at the published coordinates.
3. Please supply photos of your water sample collection method and your experimental equipment set up and any other photos from GZ that you would like to share, but please don't give away any answers. These photos must be uploaded with your online log.
Please note, this is a complex earthcache to complete, which is why it has a 4.5 star difficulty rating. With that in mind, we will not accept part answers or answers that are clearly made up. If you visit the site and perform the experiment, it's not that hard, and we will therefore reserve the right to delete logs if answers are incomplete, wildly incorrect, not provided at all, or if no photo is uploaded. This earthcache is not suited to travelling tourists as they will have nowhere to sit the water sample for 14 days without moving it.
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